Question

(a) What is an atom ? How do atoms usually exist ?
(b) What is a molecule ? Explain with an example.
(c) What is the difference between the molecule of an element and the molecule of a compound ? Give one example of each.

Answer 1

(a) Atoms are building blocks of all matter. They are the smallest particles of elements that can take part in a chemical reaction.

Atoms usually exist in combined form, in two ways:
• in the form of molecules
• in the form of ions

(b) Molecule is the smallest particle of a substance (element or compound), which has the properties of that substance. It can exist in a free state.

Example: Oxygen atoms combine and exist in the form of O₃ molecules. Solid sulphur has eight sulphur atoms joined together, and exists in the form of S₈ molecule.

(c)

Molecule of an element	Molecule of a compound
1. It consists of two or more atoms of the same element chemically bonded together.	1. It consists of two or more atoms of different elements chemically bonded together.
2. Example: Bromine exists in nature as a diatomic element (Br2), wherein two bromine atoms are covalently bonded to give a molecule of bromine.	2. Example: A molecule of nitric acid, HNO3, comprises one atom of hydrogen, one atom of nitrogen and three atoms of oxygen chemically bonded together.